"Certainly, we do not want things to get to a military conflict... but obviously if North Korea takes actions that threaten the South Korean forces or our own forces, then that would be met with an appropriate response," he said.

"If they elevate the threat of their weapons programme to a level that we believe that requires action, that option is on the table," Tillerson added.

The US Secretary of State is on a three-day Asia tour amid heightened tensions on the Korean peninsula following recent North Korea missile launches, and fears that Pyongyang is preparing for another nuclear test.

In a bid to make the current sanctions-oriented approach towards the North more effective, he urged allies and other countries to join the US-led efforts to wean Pyongyang away from nuclear ambitions, reported Yonhap news agency.

In particular, Tillerson wanted China to be active in enforcing sanctions imposed on the North for its repeated violations of UN resolutions, said the report.

"We are calling on all countries to fully implement those sanctions. We are also calling on China to fully implement those sanctions as well in compliance with UNSC resolutions that it voted for," he said.

The former oil executive didn't say much about a "new approach" towards the North that he mentioned during a press conference in Tokyo on Thursday, but reiterated that the past two decades of efforts to denuclearise the North have failed.

His visit to Seoul is also timed with the deepening spat between South Korea and China over the deployment of a US missile defence system called Terminal High Altitude Area Defence (Thaad).

China has been stepping up its retaliation against South Korea, as Seoul and Washington went ahead with their plan to install a Thaad battery on the peninsula to better counter threats from the North.

Beijing has objected to the plan, saying its powerful radar system will hurt its strategic security interests.

Tillerson called on China to refrain from retaliating against South Korea over Thaad, saying that it is "inappropriate" and "troubling".

He also stressed that the missile shield is for defensive purposes, not targeting anything other than missiles coming from the North.